THE TRANSPORTATION OF GASES 151 Velocity.—From the formula v = ^/2gH the velocity of air at standard (70°F., 29.92 in.) conditions (W = 0.075) is given by V = 4000 \/h. For any given air temperature t and pressure B, neglecting the slight effect of a variable humidity, V = 956^(460 + t)h/B. Pressure is generally expressed in inches of water or in ounces per square inch. 1 oz. per square inch « 1.732 in. of water H = 5.2h/W; P = HW, and P = 5.2k. For air at density W = 0.075, a head of 1 in. of water equals that of 69.3 ft. of air. Three pressures must be considered in a column of moving air, namely, static, velocity and impact. The first represents the compression, the second kinetic energy of the blast, and the third the total pressure or the sum of the static and velocity pressures. The static pressure in a system through which air passes is often referred to as "maintained resistance." Air Horsepower.—The horsepower in a column of moving air is a hp. = V J.P/33,000 = QP/33,000 = 5.2^/33,000 = Q.1728Ahf\/h7W- Tne horsepower required to / T \3 / w \ drive a centrifugal fan may be roughly figured as K IJTJQQ) a(n 075)' Mechanical Efficiency is the ratio of air horsepower to brake horsepower. Sometimes the air horsepower is computed by use of only the static pressure—positive, negative or both—against which the fan operates; it being assumed that static pressure represents the useful pressure and that the kinetic energy or velocity pressure is thrown away on leaving the system and entering the atmosphere. Since in most fan systems the velocity at the point of delivery into the atmosphere is considerably lower than at the fan outlet, the kinetic energy or velocity pressure which can rightfully be deducted from the impact or total head produced by the fan is only that existing at the point of leaving the system, and not the velocity pressure at the fan outlet. The impact or total head produced by the fan is more generally used in computing the air horsepower. TABLE 10.—VELOCITY OF AIR DUE TO PRESSURE (Air at 65°F.; barometer reading, 29.92 in.) Pressure, inches of water Velocity, feet per minute Pressure, inches of water Velocity, feet per minute Pressure, inches of water Velocity, feet per minute Pressure, inches of water Velocity, feet per minute 0.1 1,265 0.9 3,790 1.7 5,210 3.25 7,210 0.2 1,790 .0 4,000 1.8 5,360 3.50 7,490 0.3 2,190 .1 4,190 1.9 5,510 3.75 7,750 0.4 2 , 530 .2 4,380 2.00 5,650 4.00 8,000 0.5 2,830 .3 4,560 2.25 6,000 4.25 8,250 0.6 3 , 100 .4 4,730 2.50 6,320 4.50 8,490 0.7 3 , 350 .5 4,900 2.75 6,630 4.75 8,720 0.8 3 , 580 .6 5,050 3.00 6,930 5.00 8,950 Manometric Efficiency or pressure efficiency is the pressure developed by the fan divided by the pressure against a plane surface due to a velocity equal to the peripheral speed of the fan wheel. M = gH Id*. Manometric efficiency is defined by some engineers as 2gH /Ci2. Volumetric efficiency is not really an efficiency and might better be called volumetric capacity. It is defined as the quantity of air delivered per revolution divided by the overall cubical contents of the wheel, and is represented by the formula Vol. Eff. = Performance of Fans at Various Air Temperatures. — In Table 11 are given factors for determining the influence of the air temperature on the volume of a